The invention relates to aluminum foil having an enlarged effective surface for use in electrolytic capacitors, to a method of producing foil having such an enlarged effective surface and to electrolytic capacitors at least one of the electrodes of which consists of this foil.
A distinction can be made between what is commonly referred to as a "high voltage foil", which is formed at a voltage above approximately 200 V, which results in a thick oxide layer, and a "low voltage foil" which is formed at a voltage of not more than approximately 120 V, which results in a thin oxide layer. The desired structure of the zone of the foil below the surface is different for these classes of foil: for foils of the last-mentioned class the channel the foils of these two by means of which the effective surface is enlarged may be much narrower than those for the foils of the first-mentioned class. Channels having a diameter in the order of magnitude of the thickness of the oxide layer to be formed of course do not contribute to the capacitance. Consequently, a high-voltage foil must have a much rougher pore structure with wider channels than a low-voltage foil. It is therefore possible to obtain a much larger effective surface for foils of the last-mentioned class than for foil of the first-mentioned class.
The invention relates in particular to the "low-voltage foil".
U.S. Pat. No. 3,520,788 describes an etched high-voltage foil and a method of producing the same. This foil is of such a construction that it is provided with wide channels which extend to the middle of the foil depth, as a result of which the foil loses a substantial part of its strength compared with the unetched starting product.
The invention provides a low-voltage foil of aluminum having a value of the effective surface per unit volume of material removed higher than that previously realized.